(a) Field of the Invention
The present invention relates to an improvement of an image forming optical system and more particularly to an image forming optical system which is suited for use in endoscopes or fiber scopes and the like.
(b) Description of the Prior Art
In the field of optical instruments such as fiber scopes which receive images of objects on an end surface of an image guide composed by bundling a large number of fibers and transmit said images to the other end surface of the image guide, it is already known that quality of the images observed on the emerging end surface of the image guide is improved by vibrating relative position between the incident end surface of the image guide and the object images at a high frequency (to a degree permitting to utilize the after image effect of the eye) and at a very narrow amplitude (on the order of the thickness of the fiber) (for example, refer to S. Kapany "Fibers Optics" ACADEMIC PRESS 1967).
By utilizing this phenomenon, attempts have conventionally been made to improve qualities of images observed through fiber scopes. For example, Japanese Preliminary Patent Publication Sho No. 57-46211 discloses a fiber scope in which a permanent magnet is fixed to an outer circumference of a lens element of an objective lens system arranged before the incident end surface of an image guide and connected by way of a cushion to an AC solenoid fixedly attached to the image guide. Said fiber scope improves image quality by adopting such a composition as to vibrate an image of object on the incident end surface of the image guide by vibrating said lens element under the action of an alternating magnetic field produced by applying AC current to an AC solenoid. On the other hand, the same composition is adopted also on the emerging side of the image guide to vibrate the image emerging from the image guide in synchronization with the vibration on the incident side, thereby preventing the observed images from being blurred due to the vibration of said lens element.
In the conventional example described above wherein said lens element is vibrated on both the sides of an axis perpendicular to the optical axis, however, a surplus space must preliminarily be reserved around said lens element in its diametrical direction since the edge line on the outer circumference of said lens element protrudes in the direction of the outside diameter thereof. Accordingly, it posed a problem that the distal end of the fiber scope was inevitably thickened.
Further, there are widely used so-called electronic fiber scopes which use solid-state image sensors in place of said image guide. For such electronic fiber scopes which require very small image sensors, it is a very important problem how to enhance resolution. Japanese Patent Publication Sho No. 56-40546 discloses a method to solve the problem. This method improves resolution by arranging plural number of image sensors at positions deviating from the optical axis for a distance corresponding to 1/2 or 1/3 of the pitch reserved between two neighboring picture elements on the plane perpendicular to the optical axis of the imaging lens so that the relative positions are different from each other between one row of the picture elements and the other row of the picture elements and obtaining the information corresponding to the areas between the picture elements of one image sensor from the other image sensor.
In the fiber scopes, however, it is difficult from viewpoint of space, in the first place, to arrange plural number of solid-state image sensors, beam splitters for leading lights thereto, etc. in the distal ends. In addition, the fiber scope of the type described above requires preliminarily arranging plural number of solid-state image sensors at positions deviating from each other for the purpose of adjusting deviating distance of images to a predetermined value, thereby requiring high positioning precision for said sensors and making it tedious to assemble the image sensing system.